When Max von Laue showed that X-rays are diffracted in crystals and form characteristic patterns on photographic film, he proved in a single experiment that X-rays are wave-like in nature, and that crystals have a lattice-like structure. What wasn’t clear was whether the structure of the crystal and the wavelength of X-rays had any influence on the diffraction pattern. The connection, with its enormous practical implications, was established by the father and son partnership of William Henry Bragg and William Lawrence Bragg, for which they were awarded the Nobel Prize in Physics in 1915.
The key to the Braggs’ breakthrough is the fact that crystals are made of regular, repeating patterns of atoms, like oranges packed in a box. Rather than considering the dreadful complexity of each individual atom’s contribution to the X-ray diffraction pattern, the Braggs’ hypothesis was that one could mathematically predict the diffraction pattern from reflections from each successive plane of atoms within the crystal.
William Lawrence Bragg proposed a simple but powerful equation – which became known as Bragg’s law – showing the connection between the wavelength of the X-rays, the distance between the planes and the angle at which the X-rays are reflected. At the same time, William Henry Bragg designed the X-ray spectrometer, a device that could examine the reflections of X-rays from crystals. Together the two scientists used the spectrometer to analyse the structure of several salts and small molecules, establishing fundamental mathematical relationships between an X-ray diffraction pattern and the dimensional arrangement of atoms in a crystal that produced the pattern.
Their findings created the new science of X-ray crystallography, making it possible to determine molecular structures from the crystal form of a compound. William Lawrence Bragg was 25 years old when he was awarded the Nobel Prize, and to this day remains the youngest ever Laureate.*
* Since 2014, Nobel Peace Prize Laureate Malala Yousafzai is the youngest ever Laureate.